Fuel cells convert, physically separated at two electrodes, a fuel and an oxidizing agent to electricity, heat, and water. Hydrogen, methanol or a gas rich in hydrogen can serve as the fuel, oxygen or air can serve as the oxidizing agent. The process of energy conversion in the fuel cell is distinguished by a distinct lack of pollutants and by very high efficiency. For this reason, fuel cells are becoming increasingly important for alternative driving concepts, energy supply systems for buildings, as well as portable applications.
PEM fuel cells are constructed from many fuel cell assemblies stacked on top of each other. They are electrically connected in series to increase the operating voltage. The so-called membrane electrode assembly (MEA) forms the core of a PEM fuel cell. The MEA consists of the proton-conducting membrane (polymer electrolyte or ion membrane), the two gas diffusion layers (GDLs or “backings”) on the sides of the membrane and the electrode layers situated between the membrane and the gas diffusion substrates. One of the electrode layers is provided as an anode for the oxidation of hydrogen, and the second electrode layer is provided as a cathode for the reduction of oxygen.
Depending on their specification and field of application, these catalyst components in fuel cell stacks contain considerable amounts of noble metals such as platinum, ruthenium, palladium and others. For example, a 50 kW PEM stack, as it is currently used for portable applications in automobiles, contains about 50 to 100 g of platinum (i.e. about 1 to 2 g of platinum per kW). Therefore, a large-scale introduction of fuel cell technology into automobiles with a large number of units would require considerable amounts of platinum, at least for the first generation of vehicles. Moreover, a recovery process for the noble metals bound in the fuel cell stacks would then have to be provided to secure the noble metal cycle and thus guarantee the noble metal supply.
The fuel cell components that have to be reprocessed in order to recover the noble metals are comprised of various materials.
The polymer electrolyte membrane consists of polymer materials that conduct protons. Hereinafter, these materials will also be briefly referred to as ionometers. Preferably, a tetrafluoroethylene/fluorovinylether copolymer having sulfonic acid groups is used. This material is, for example, distributed by DuEont under the tradename Nafion®. However, other ionomer materials, such as doped sulfonated polyether ketones or doped sulfonated or sulfinated aryl ketones or polybenzimidazoles, can be used as well. Suitable ionomer materials are described by O. Savadogo in “Journal of New Materials for Electrochemical Systems” I, 47-66 (1998). For use in fuel cells, these membranes generally have to have a thickness between 10 and 200 μm.
In addition to the proton-conducting, fluorine-containing polymer (e.g. Nafion®), the electrode layers for the anode and the cathode comprise electrocatalysts, which catalytically promote the corresponding reactions (oxidation of hydrogen and reduction of oxygen, respectively). Metals of the platinum group of the periodic table of the elements are preferably used as catalytically active components. Often, so-called support catalysts are used wherein highly disperse forms of the catalytically active platinum group metals are applied to the surface of a conductive support material, for example carbon black.
Generally, the gas diffusion layers (GDLs) consist of carbon fiber paper or carbon fiber fabric, which are usually rendered hydrophobic by fluorine-containing polymers (PTFE, polytetratluoroethylene, etc.). They allow easy access of the reaction gases to the reaction layers and good dissipation of the cell current and the water formed.
In the construction of fuel cell stacks, GDLs and MEAs are stacked on top of each other using so-called bipolar plates. Usually, this is done in the following order: End plate—GDL (anode)—CCM—GDL (cathode)—bipolar plate—GDL (anode)—CCM—GDL (cathode)—bipolar plate (etc.)—end plate. Depending on the desired performance range, up to 100 MEAs are stacked on top of each other. The bipolar plates usually consist of conductive carbon, preferably graphite. They comprise milled channels in a specific pattern which provide the gas supply (fuel gas to the anode and air to the cathode) in the stack. In the recovery of noble metals from PEMFC stacks, the bipolar plates can usually be separated from the stack when it is dissembled and recycled. However, there are also processes wherein the entire stack (including the bipolar plates) is subjected to the recovery process.
In addition to large-scale production processes for catalyst-coated membranes (CCMs), for catalyst-coated gas diffusion substrates (CCBs) as well as for membrane electrode assemblies (MEAs), the commercialization of PEM fuel cell technology above all also requires large-scale and efficient processes for the recovery of noble metals from these components. Only the application of such processes and the associated use of noble metals from the secondary cycle will render fuel cell technology economically and ecologically viable. The provision of appropriate recovery processes provides the prerequisite for fuel cell aggregates for mobile, stationary and portable applications to come on the market in high numbers.
Heat treatment processes, in particular pyrometallurgical processes, for the reprocessing and concentration of residual substances (“refuse”) containing noble metals have been known for a long time. Shaft furnaces, refining furnaces or converters, electric furnaces (plasma or electric-arc furnaces), as well as gas-heated or electrically heated crucible furnaces are the centerpieces of the processes employed world-wide. The shaft furnace process is suitable in particular for the reprocessing of refuse rich in silver, with lead being used as a collecting metal for the noble metal. In addition to the crude lead containing the noble metal, copper matte and a slag are formed, which contains the non-metallic components of the refuse. Additives such as limestone, magnesium oxide, sand and calcinated pyrite are used to adjust the viscosity of the liquid slag melt (cf. Lüger, Lexikon der Hüttentechnik, Deutsche Verlagsanstalt Stuttgart, 1963, pages 548 to 553).
Furthermore, conventional combustion processes are known for the concentration of noble metals from catalysts. Residues of catalysts having combustible carbon supports (such as e.g. Pd/activated carbon) are burned in gas furnaces and the noble metal-containing ash is reprocessed. Normally, the noble metal concentration after incineration is sufficiently high to allow direct development using wet chemical methods (cf. in this connection C. Hagelüken, Edelmetalleinsatz und—Recycling in der Katalysatortechnik, Erzmetall 49, No. 2, pages 122 to 133 (DZA Verlag für Kultur und Wissenschaft, D-04600 Altenburg).
However, there are only few examples in the literature regarding the reprocessing of fuel cell components containing noble metals.
U.S. Pat. No. 5,133,843 suggests a method for reprocessing or “rejuvenating” an ionomer membrane coated with noble metals, which comprises dissolving the noble metals in aqua regia. The ionomer membrane can then be re-used in fuel cells.
JP 11/288732 describes a method for recovering components for fuel cells, wherein the membrane electrode assemblies are treated with a solvent that dissolves the fluorine-containing ionomer or the membrane. The fluorocarbon polymer is thereby separated from metallic catalysts and other insoluble components. A disadvantage of this method is the use of organic solvents which pose problems with respect to combustibility, industrial safety, environmental damage and toxicity. The subsequent reprocessing of the fluorine-containing catalyst components is not described.
It was an object of the invention to provide a process for the concentration of noble metals from fluorine-containing fuel cell components that overcomes the disadvantages described.
This object is achieved by the process according to claim 1. The dependent claims relate to preferred embodiments.